This invention relates generally to boring tools and more particularly to so-called “spade bits,” which are typically used with a drill for drilling holes in wood.
Spade bits are widely used for boring holes between approximately ¼″ and 1½″ diameter in wood and similar soft materials, because they are fast, true cutting, resharpenable, and relatively inexpensive. Their name derives from the shape of the spadelike blade or cutter, in contrast to the spiral shape of augers and twist drills. The blade, which is usually forged integrally from the shank, is relatively thin and flat and may have a width several times the diameter of the shank.
One known spade bit is disclosed in U.S. Pat. No. 4,682,917. A partial cross-section of this drill bit is shown drilling a bore in FIG. 6. This bit 11 includes a blade 3 shoulder flute 5 adjacent the cutting edge of each shoulder. The flute has a cross-sectional configuration having an inner surface portion 7 and an outer surface portion 9 where the outer portion and inner portion consist of planar surfaces that meet at an angle. The shoulder flutes preferably do not extend all the way to the outer sides of the blade but rather stop inwardly of the sides of the blade.
The inventor of the present drill bit determined that because the surface portions 7 and 9 comprise substantially planar surfaces that meet at a relatively sharp angle, the approximate path of travel of chips generated by this bit is as shown by the arrows in FIG. 8. Specifically the chips curl up face 9 until they strike face 7 at which point they are reflected at a relatively severe angle almost perpendicular to the direction of travel of the bit, represented by arrow C. The chips are trapped in the bore 13 where they are swirled around by the drill bit blade 3 as the drill bit rotates until the chips are eventually ejected from the bore.
It has been determined that moving the chips within the bore 13 by blade 3 slows the drilling rate and requires energy. Thus, not only is the drilling rate slower than optimal, the amount of energy used to drill the bore is increased. The use of excess energy to drill the bore can present a significant issue because many drills are battery powered such that the need for additional energy adversely affects the charge life of the battery resulting in the need for more frequent battery charges.
Thus, there is a need in the art for a bit that increases the drilling rate and that requires less power.